Novel compositions with controlled electrical properties

ABSTRACT

An electrical conductor shielded with an insulating coating comprising a semi-conductive material of a polymer having a dielectric constant of at least 2.0 and a filler selected from the group consisting of electrically conductive metals and their alloys, said filler having a particle size range of about 0.05 to about 50 microns, said particles having an approximately linear distribution of particle size with an average particle size of about 1 to about 20 microns.

United States Patent [151 3,666,876

Forster 1 51 May 30, 1972 541 NOVEL COMPOSITIONS WITH 3,096,210 7/1963BOOl'lSll'a .1 ..174/120 sc CONTROLLED ELECTRICAL 3,287,489 11/1966Hvizd, Jr. ..174/ 102 SC P OPERTIES 2,377,153 5/1945 Hunter et al...174/120 SC X 72 inventor: Eric 0. Forster, Seotch Plains, NJ. FOREIGNPATENTS OR APPLICATIONS [73] Assignee: Esso Research and EnglneerlngCompany 824,861 12/1959 Great Britain ..174/102 SC [22] Filed: 1970Primary Examiner-Lewis H. Myers [21] App]. No.: 55,776 AssistantExaminer-A. T. Grimley Related us. Application Dam Attorney-Chasan andSinnock and'A. Lagani, J r.

[63] Continuation-impart of Ser. No. 678,655, Oct. 27, ABSTRACT 1967abandoned An electrical; conductor shielded with an insulating coatingcomprising a semi-conductive material of a polymer having a [52] usI'M/I02 ag 321% dielectric constant'of at least 2.0 and a fillerselected from the [51] Int. Cl. ..H0lb 9/02 group consisting ofelectrically conductive metals and their 58 Field 61 Search ..174/36,102 SC, 105 sc, 106 sc, W said having Panicle Size range of 4 120 SC 120R 12 7 5 2 12 2 about 50 microns, said particles having an approximatelyI 230 232 linear distribution of particle size with an average particlesize I of about 1 to about 20 microns. 56 R f C'ted 1 e erences l i 4Claims, 4 Drawing Figures UNITED STATES PATENTS 3,433,891 3/1969 Zysk eta] ..174/102 SC Patented May 30,1972 I 3 Shun-Shoo 1 FIG. I

' HIGH VOLTAGE CURRENT CHARACTERISTICS OF METAL FILLED SYNTHETIC RUBBERS1 I I I I I l l I. EPT 4504 FILLED WITH AI POWDER I 2. ENJAY BUTYL 035FILLED WITH AI POWDER 3. EPT4505 FILLED WITH Fe POWDER- 4. ENJAY BUTYL035 FILLED WITH Fe POWDER All in form of 70 mil pad 5. SIMPLEX TAPE(20mil) 6. NON-FILLED ELASTOMER vi a E 'DECREASING 0-" I VOLTAGE 2 u: 44 cc m U INCREASING c VOLTAGE 2 6/ 1 I I 4L, -6 I I 1 1 l I I I IAPPLIED VOLTAGE V Patented May 30, 1972 3,666,876

' s Shuts-Sh. 2

FIG. 2

HIGH VOLTAGE CURRENT CHARACTERISTICS OF HEAT SHRINKABLETUBING-VULCANIZED PAD I 1 I l I I I I 40% OF (70% POLYETHYLENE so ENJAYEPR 404) a 60% 500 MESH IRON POWDER PAD THICKNESS 0.032"

RUNS ON 2 PADS AVERAGE o VOLTAGE INCREASING [1 VOLTAGE DECREASING logCURRENT, amps.

I IIIIII I I I I I I I I O 2000 4000 6000 V 8000 I0000 APPLIED VOLTAGE5. 0.13m!" lnv Ior Patented May 30, 1972 3 Shuts-Sh! S E. 0. F yrs/erInventor NOVEL COMPOSITIONS WITH CONTROLLED ELECTRICAL PROPERTIESCROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of US. Pat. application, Ser. No. 678,655, filedOct. 27, 1967 now abandoned.

BACKGROUND OF INVENTION Elastomers such as butyl rubber, ethylenepropylene copolymers, ethylene propylene terpolymers (EPDM) and SBR havebeen used for wire insulation and generally as electrical insulatorsbecause of their inability to conduct electrical currents. In manyapplications involving high electrical stress of kv or greater, itbecomes difficult if not impossible to join cables without enhancingelectrical breakdown at the splice.

When high voltage electric current flows through a conductor, a field isset up in the insulation surrounding the conductor. In order to splice asecond conductor onto the first conductor, it is necessary to remove aportion of the insulation. Reinsulating the stripped wire with the sametype of insulation is unsatisfactory since the discontinuity between theinsulations results in field breakdown and arcing across the insulatedsplice. Similarly, at terminal points where the insulation of the cablehas been removed for contact purposes, it is necessary to protect thetenninal and base wire from corona discharges to ground. Most insulatingmaterials are inadequate, at conventionally used thicknesses, inpreventing such discharge and breakdown. Theory predicts that one couldavoid these difficulties if one could produce a material, the electricalconductivity of which is a function of applied stress provided that suchdependency would be reversible.

High voltage conductors are commonly insulated with crosslinkedpolyethylene. This material, however, has the disadvantage that it isprone to bubble formation during curing and hence breakdown at highvoltages. Further heat generation within the polymer as a result of thehigh voltage field surrounding the conductor causes thermal degradation.If the acceleration potential, i.e., voltage drop, across the insulatorcould be reduced, arcing and breakdown could substantially beeliminated.

Semi-conductive tapes have been formed by incorporating silicon carbideinto PVC polymers. These tapes have been useful in avoiding fieldbreakdown in splices in high voltage lines. Pseudo-semi-conductive tapeshave also been formed by incorporating carbon black in all types ofpolymers, including suitably crosslinked polyethylene. These types oftapes have found wide use as separators between the central metallicconductor and the insulation layer particularly where the metallicconductor is formed by strands of metal lines. Because of their ratherlow resistance value, these carbon black filled polymers are also usedat times as anelectrical ground at the outside of a cable since it isless prone to corrosion than would be metallic grounds.

SUMMARY OF INVENTION It has surprisingly been found that semi-conductorssuitable for use as insulating materials for high voltage line splicesmay be prepared by dispersing particles of metal and magnetic ornon-magnetic alloys, into a polymer, the polymer preferably having adielectric constant of at least 2.0. Additionally, by controlling theconcentration of the particles as a function of thickness of theinsulator, an insulation material may be prepared having controlledelectrical properties that are not subject to breakdown at highvoltages.

In the practice of this invention metal or alloy particles are dispersedin a polymer, preferably an elastomer, and compounded with variouscuring agents and bonding agents. The compounded material is thenextruded onto an electrical conductor and cured in place.

The compositions of this invention may be formed into tapes for wrappingsplices. Similarly, heat shrinkable tubes may be formed to be placedover splices and shrunk into place.

In a particularly preferred embodiment, these compositions are used toprepare insulated conductors having unusually high breakdown voltagecomprising a conductor coated with multiple layers of insulation, eachlayer, in an outward direction from the conductor, containing a lesseramount of filler than the previous layer. The coating nearest theconductor may contain as much as wt. percent filler. The outermost layerpreferably contains about 20 to 40 wt. percent filler.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 presents a comparison of highvoltage current characteristics for metal filled synthetic rubbers.

FIG. 2 is a plot of the high voltage current characteristics of heatshrinkable tubing.

FIG. 3 illustrates the structure of multilayered insulated conductors.

DETAILED DESCRIPTION Throughout the specification and claims, the termelectrically conductive metals means those materials which have aresistivity of between about 10 and about 10 ohm-centimeters. Typical ofsuch electrically conductive metals are magnesium, the metals of GroupVIII of the Periodic Table of the Elements (E. H. Sargent & Co., 1966)such as iron, cobalt, nickel and platinum, the metals of Group 1-8 ofthe Periodic Table such as silver, gold and copper or the metals ofGroup II-B of the Periodic Table of the Elements such as zinc andaluminum. Illustrative of the magnetic materials suitable for use in thepractice of this invention are alloys such as Fe-Ni, Co-Ni, Cu MnAl, andCu,MnSn. Illustrative of suitable nonmagnetic materials are alloys suchas Cu-Sn, Cu-Zn, and Al- Mg.

The term semi-conductor" as used in the specification and claims means amaterial whose conductivity is a non-linear function of the appliedstress, i.e., voltage potential, and which has a resistivity of between10 to 10 ohm-centimeters. While metals tend to have increasingresistivity with increasing temperature, the semi-conductors show adecrease in resistivity with increase in temperature.

Under low voltage stresses, the semi-conductors act as insulators andconduct no electricity. As the voltage increases to the order of 10l00kilovolts, the semi-conductors begin to conduct electricity, theconductance being a function of the stress applied and increasing as theapplied stress increases.

This behavior is quite unique and cannot be achieved with carbon blackfiller polymers. The reason for this difference is believed to resultfrom the different structure of metallic fillcrs and carbon black. Thelatter forms structural units, i.e. chains of particles which cannot bebroken up under normal mixing conditions. Metal fillers as contemplatedin this invention do not possess such a structure but are known toconsist of individual, discrete particles unless they are magnetic inwhich case they can form agglomerates which can be readily broken up innormal mixing operations. The basis of this invention therefore appearsto be the formation of a uniform dispersion of discrete metal particlesall essentially insulated from each other by very thin polymer films. Asthe voltage increases beyond a certain critical value, the electronsemanating from one particle can tunnel through the thin film into thenext particle and thus cause a rapid increase in current flow withoutcausing permanent dielectric breakdown. In the case of the carbon blackfilled polymers, it has been noted that under similar conditions,permanent breakdown results. This observationis attributed to theinability of forming uniform dispersions of carbon black in polymers sothat the number of direct carbon-carbon contacts and the separation ofcarbon chains from each other are hard to control. In these carbonfilled polymers, sudden voltage surges lead to heavy current flowthrough the few existing contacts and hence to overheating which resultsin the formation of carbonization of the adjacent polymeric material.The net result of such a surge is the formation of highly conductivetracks and permanent damage to the material.

Any polymer which may be readily extruded or coated onto an electricalconductor is suitable for use in the practice of this invention.Preferably, the polymer has a dielectric constant greater than 2.0.Illustrative of such polymers are polyethylene, butyl rubber,ethylene-propylene copolymers, ethylene-propylene terpolymers (EPDM),styrene butadiene rubber, polyvinyl chloride and mixtures thereof.

The expression butyl rubber" as employed in the specification and claimsis intended to include copolymers made from a polymerization reactionmixture having therein about 70 to about 99.5 percent by weight of anisoolefin which has about 4 to 7 carbon atoms and about 30 to 0.5percent -by weight of a conjugated multiolefin having about four -toabout 14 carbon atoms. The resulting copolymer contains 85 to 99.5percent of combined isoolefin and 0.5 to 15 percent of combinedmultiolefin. The term butyl rubber" is described in an article by R. M.Thomas et al. in Industrial Engineering and Chemistry, vol. 32, pages1,283 et seq., Oct., 1940.

The butyl rubber generally has a Staudinger molecular weight between20,000 to 500,000; preferably about 25,000 to about 200,000; especially45,000 to 60,000; and a Wijs Iodine No. of about 0.5 to about 50;preferably 1 to 15. The preparation of butyl rubber is described in U.S.Pat. No. 2,356,128 which is incorporated herein by reference.

Typical of such butyl rubber is Enjay Butyl 035 (Enjay Chemical Co.) apolymer having a Mooney viscosity at 212 F. of about 38-47 and a molepercent unsaturation of about 0.6 to about 1.0.

The term EPDM is used in the sense of the definition as found inASTM-D-l418-64 and is intended to mean a terpolymer containing ethyleneand propylene in the backbone and a diene in the side chain.Illustrative methods for producing these terpolymers are found in U.S.Pat. No. 3,280,082 and British Pat. No. 1,030,989 which are incorporatedherein by reference. Any EPDM may be used in the practice of thisinvention.

The diene monomer is preferably a nonconjugated diene. Illustrative ofthese nonconjugated diene monomers which may be used in the terpolymer(EPDM) are hexadiene, dicyclopentadiene, ethylidene norbornene,methylene norbomene, propylidene norbornene, and methyltetrahydroindene. The particular diene used does not form a criticalpart of this invention and any EPDM fitting the above description may beused. A typical EPDM is Vistalon 4504 (Enjay Chemical Co.) a polymerhaving a Mooney viscosity at 212 F. of about 40 prepared from a monomerblend having an ethylene content of about 6 wt. percent and anon-conjugated diene content of about 3.3 wt. percent.

The electrically conductive metal particles and ferromagnetic particlesused in this invention should have a particle size fine enough to passthrough a 425 mesh screen, i.e. less than 50 microns, preferentiallythrough a 500 mesh screen, i.e. less than 45 microns. The particle sizemay vary between about 0.05 to about 50 microns, more preferably about0.05 to about 45 microns, most preferably about 0.05 to 30 microns. Theaverage particle size is preferably 1 to about 30 microns; morepreferably about to about microns. To produce satisfactory results, interms of elongation, tensile strength and electrical properties, it isadvisable to use a powder of broad but uniform particle sizedistribution. This can be achieved by intentionally mixing powdersproduced by grinding particles and sieving them through a 500 meshscreen, for example, and admixing to this material a fraction ofcolloidally produced powder (i.e. ca 0.05 micron) of the same material.The methods for producing colloidal particle size powdered metals iswell known to the art.

By controlling the ratio of the two fractions, one can maximize eitherelectrical or mechanical properties or produce a compromise whichsatisfies the particular needs under consideration. The concentration ofthe electrically conductive metal powders in the polymer matrix may befrom about 20 to about 90 wt. percent, preferably from about 40 to about80 wt. percent, more preferably from 50 to 70 wt. percent.

The terms filler" or fil1ed" as used throughout the specification andclaims refer to the electrically conductive particles of metal andmagnetic or non-magnetic alloys of this invention.

In the practice of this invention. it has been found advantageous to usea coupling agent which serves to bring about a better bond between themetal particles and the polymer matrix. Preferably, the coupling agentis an unsatu rated organosilane which is employed in amounts rangingfrom about 0.1 to about 5, preferably about 1 to about 4 parts perweight per hundred parts of polymer mix. Although the metal particlesmay be treated with the organosilane rather than adding the organosilaneto the polymer mix, the latter technique has been found to be moreconvenient.

The term organosilane as employed herein includes the silane, itssilanols (the corresponding partially or completely hydrolyzed forms ofthe silane), its siloxanes (the corresponding condensation products ofthe silanols) and mixtures thereof. The organosilane may be representedby the formula:

- wherein R is a C -C radical containing vinyl type unsaturationselected from the group consisting of alkenyl styryl, alkenyl alkyl,alkenoloxalkyl; X is selected from the group consisting of hydroxyl,alkoxy acyloxy; R and R are independently selected from the groupconsisting of hydroxyl, methyl, alkoxy, acryloxy and R Nonlimitinguseful compounds which may be employed are the following: vinyltri(beta-methoxy-ethoxy)-silane, vinyl triethoxy silane, divinyldiethoxy silane, allyl triacetoxy silane; in place of the vinyl andallyl groups of the above named compounds, the corresponding styryl,acryloalkyl, methacryloalkyl, acryloxy propyl and methacryloxy propylcompounds may be used. All of the silanes are convertible into theuseful corresponding silanols by partial or complete hydrolysis withwater. The preferred organosilanes are gamma-methacryloxypropyltrimethoxy silane and vinyl tri-beta-methoxyethoxy silane.

The following examples serve to further illustrate how the processes ofthis invention may be carried out as well as the benefits derived fromits use.

EXAMPLE 1 A series of compounded mixes designated as samples A-F wereprepared using Vistalon 4504 and Enjay Butyl 035 as the polymers. Themetal or ferromagnetic powders used were iron powder, iron oxide powderand aluminum powder. The exact formulation of these blends is shown inTable I.

In the preparation of these blends, the polymer was milled on a cool(i.e. below 130 F.) mill and allowed to band. The metal powder ormagnetic material was gradually added to the banded polymer at a ratesufficient to prevent destruction of the polymer band. After the fillermaterial was added, the other compounds were then added starting withthe silane. The vulcanizing agents were added last and the mixture wasallowed to blend for about 20 to about 30 minutes. The samples were thenpress cured at 320 F. for 20 minutes to form 70 mil pads, the physicalproperties of which are shown in table I. The electrical properties ofthese filled metal elastomers are shown in Table l. The electricalproperties of these filled metal elastomers are shown in Table II.

It will be noted that as the applied voltage is increased from volts to3,500 volts, the current increases. It is noted further that there is nohysteresis effect, that is in going from the lower voltage to the highervoltage and back to the lower voltage, there is essentially no change inelectrical properties.

TABLE I.-POWDERED METAL FILLED ELASTOME RS g A B O D E F Enjay Vistalon4504 100 100 100 Enjoy buty1035 100 100 100 Zinc oxidc 5 5 .5 5 5 5 ironpowder 500 mesh 200 200 0.5 fired iron oxide powdcr 200 200 Aluminumpowder 200 200 A-172 silane 1 2 2 2 2 2 2 AgeRitc resin D 2 .1. 5 1.5 1. 5 ERD 90 (Pba04 dispersed). 5. 6 5.6 5. 6 5.6 5.6 5.6 Drimix TAC 32 2 2 Di-Cup 40 HAF 7 7 7 Stearlc acid .5 0. 5

5 5 4 4 2. 85 Press cures 207320 Hardness Shore A 64 71 g 80 48 45 .63

100% modulus, p.s.i 220 220 130 so 160 Tensile strength, p.s.i. 380 260370 780 140 320 Percent elongation 280 210 80 790 600 710 1 Vinyl trls(methoxy ethoxy) silane.

1 Polymerized trlmethyl dihydroquinoline.

3 25% dispersion of triallyl cyanurate on mlcrocel.

4 Dlcumyl peroxide 40% on carbon black.

4 Parafiin wax having a melting point of 133 F.

Benzothiazyl disulfide (R. 'l. Vanderbilt (30.).

7 P-quinone dloxime.

composition of the polymer blend contained 70 parts polyethylene, 30parts ethylene-propylene rubber. The ingredients listed above were addedto the Banbury mixer to according to the following time schedule:

Cup R using a cool mill.

The finished blend was vulcanized at 500 psi and 320 F. for 20 minutes.The resulting 0.032 inch pad. was tested electrically, the results ofwhich test are shown in FIG. 2. it will be noted that the product hadexcellent reversible response to electrical stresses. The material had abreakdown voltage. of about 120 kilovolts per centimeter, whereas theunfilled material is known to fail at stresses below 100 kilovolts percentimeter.

TABLE III-ELECTRIC PROPERTIES OF METAL FILLED ELASTOMERS A B C D E F EPTEPT EPT Butyl Butyl Butyl Fe Al F8103 Fe Al F 03 4. 6 9. 83 5. 2 4. 8

1. 9X10 1. 2X10 10' l0- 1. 1X10 4X10' 5. 8X10 7Xl0 3. 8X10- 4Xl0' 6X10'4. 6X10- 10- 3X10 6X10' 6. 2X10 2X10' 1. 2X10' Ill- 10' 1. 1X10 10 1Measured with a Keithley 610B electrometer using a N.J.E. re ulatedpower supply with continously variable voltage output from 0 to 30,000VDC, shielded connectors an a 2" diameter measuring electrode. Sampleswere measured at 100, 200, 500, 1

000, 1,500, 2,000, 2,500, 3,000 and 3,500 volts, keeping the voltage atthese values or at least five minutes. The voltage was reduced in thesame manner. Each sample was subjected to at least 3 uch cycles.

These data are shown graphically in FIG. 1. Curve 5 is a typical curvefor a Simplex tape (a silicon-carbide containing PVC) of mil thickness,whereas curve 6 is a typical curve for an elastomer having no filler. itwill be noted that the nonfilled material and those filled withnonconductive mo, show only the current gain as a function of appliedvoltage predictable from ohms law. Hence, the electrical stress orvoltage drop across any insulating material made of any such materialswill be very large, whereas in contrast, the electrical stress isdecreased in the semi-conductors of this invention due to increasedcurrent flow.

EXAMPLE 2 Formulation Parts Master batch 60 Polyethylene 4O AgeRite D0.4 Zinc Oxide 5.0 ERD 90 (Pb O 90%) 5-6 Drimix TAC dispersion oftriallyl cyanurate on microcel) 2 500 mesh iron powder 150 A-l 72 silane(vinyl tris methoxyethoxy)silane) l Di-Cup R (Dicumyl peroxide) Mixingwas accomplished in a Banbury blender according to the time/temperatureschedule shown below. The final It is often desirable to shield currentcarrying conductors such as television antennas and auto electrical coilwirings and plug wirings to prevent disruptive efiects from straycurrents.

' Such shielded conductors generally have a central conductor coveredwith an insulator and an outer conductive shield which is connected toground. A similar shielded material having equivalent shieldingprotection properties may be prepared as shown in FIG. 3a. The centralconductor, 1, is coated with an insulating composition, 2, whichcomprises the semi-conducting material of this invention containingabout 25 percent of the tiller particles. The succeeding layers, 3, 4and S, are composed of the semi-conducting material of this invention,each succeeding layer having a higher percentage of filler. For example,layer 3 may have from about 30 to 40 wt. percent filler, layer 4 mayhave 50 to 60 wt. percent filler and layer 5 may have to wt. percentfiller. The outer layer, 6, contains about wt. percent filler and inthis case the filler material may consist of particles larger than thosespecified for the semiconducting material of this invention and may beas large as 200 microns in order to insure that the outer coating isconductive. The term conductive coating as used in the specification andclaims is one which has a resistivity less than about 10ohm-centimeters. The particle size range in the conductive coating mayvary from about 0.05 to about 200 microns. Where the larger particlesare used, the filler may be present at about 80 to 90 wt. percent. Thisouter coating acts as a shielding and is connected to ground just as inthe prior art shielded terminals the outer sheath is connected toground.

Using the semi-conductive compositions of this invention, it Eispossible to prepare an insulated electrical conductor which has anunusually high breakdown voltage and the surprising characteristic thatcurrent leakage may occur without actual damage to the insulatingmaterial. Referring now to FIG. 3b, a center electrical conductor, 1, iscoated with a layer of the semi-conducting material, 2, of thisinvention having about 5 80-90 wt. percent filler particles. Thesucceeding layers, 3,

and 5, each have a lesser amount of filler, for example, layer 3 hasfrom about 70 to 80 percent filler, layer 4 has from about 50 to 60percent filler and layer 5 has about 25 to 50 percent filler.

The semi-conductive characteristics of the coating permit a limitedcurrent flow within the insulator at high voltage stresses and thereforereduces the acceleration potential across the insulator, thereby givingthe material an unusually high breakdown voltage. As has been pointedout earlier, the current flow under these conditions involves manyparticles all separated by very thin films so that in no individual pairof large number of electrons is flowing and hence no significantlocalized heating occurs. As the voltage reaches very high values underan overload, tunneling will become also probable between particleshaving larger separations, thus preventing excessive electron passagethrough a limited number of tunneling positions. Thus, in the event ofovervoltage loads, a sufficient current flow may occur to prevent actualrupture of the insulator as would normally be. the case in conventionalinsulation materials.

Since many different embodiments of this invention may be made withoutdeparting from the spirit and scope thereof, it is to be understood thatthe present invention is not limited to the embodiments specificallydisclosed in the specification thereof.

What is claimed is:

l. A shielded electrical conductor comprising:

a. an electrically conductive conductor;

b. a first insulating semi-conductor composition coating surroundingsaid conductor comprising: 1. a polymer having a dielectric constant ofat least 2.0,

and 2. a filler selected from the group consisting of electricallyconductive metals and their alloys, said filler having a particle sizerange of about 0.05 to about 50 microns, said particles having anapproximately linear distribution of particle sizes with an averageparticle size of about 1 to about 30 microns, wherein said firstinsulating semi-conductor coating comprises about to about 40 wt.percent, based on the total composition of filler;

c. a series of additional insulating coatings laid one upon the otherover the first insulating coating, said additional coatings comprisingthe semi-conductor compositigrr ot:

8 (b) (l) and (2); each succeeding coating containing about 10 to about30 wt. percent more filler than the preceding coating up to a maximum ofabout 90 wt. percent based on the total composition; and

d. a final outer conductive coating.

2. The composition of claim 1 wherein the final outer coating comprises:

a. a polymer having a dielectric constant of at least 2.0; and

b. about to about wt. percent based on the total composition of a fillerselected from the group consisting of elecuically conductive metals andtheir alloys, said filler having a particle size range of about 0.05 toabout 200 microns.

3. The composition of claim 2 wherein the filler is present at about 90wt. percent and has a particle size range of about 0.05 to about 50microns, said particles having a linear distribution of particle sizewith an average size of about 1 to about 30 microns.

4. An insulated conductor which comprises an electrically conductiveconductor insulated with a semi-conductor coating compositioncomprising:

a. a first layer of the semi-conductor composition comprismg: l. apolymer having a dielectric constant of at least 2.0,

and 2. a filler selected from the group consisting of electricallyconductive metals and their alloys, said filler having a particle sizerange of about 0.05 to about 50 microns, said particles having anapproximately linear distribution of particle sizes with an averageparticle size of about 1 to about 30 microns, wherein said firstinsulating semi-conductor coating comprises about 70 to about 90 wt.percent based on the total composition of filler;

b. a series of additional layers of semi-conductor composition laid oneupon the other over said first layer, said additional layers comprisingthe composition of (a) (l) and (2); each of said additional layerscontaining about 10 to about 30 wt. percent less filler than thepreceding layers; and

c. a final outer layer of said semi-conductor composition containingabout 20 to about 40 wt. percent filler.

2. a filler selected from the group consisting of electricallyconductive metals and their alloys, said filler having a particle sizerange of about 0.05 to about 50 microns, said particles having anapproximately linear distribution of particle sizes with an averageparticle size of about 1 to about 30 microns, wherein said firstinsulating semi-conductor coating comprises about 20 to about 40 wt.percent, based on the total composition of filler; c. a series ofadditional insulating coatings laid one upon the other over the firstinsulating coating, said additional coatings comprising thesemi-conductor composition of (b) (1) and (2); each succeeding coatingcontaining about 10 to about 30 wt. percent more filler than thepreceding coating up to a maximum of about 90 wt. percent based on thetotal composition; and d. a final outer conductive coating.
 2. Thecomposition of claim 1 wherein the final outer coating comprises: a. apolymer having a dielectric constant of at least 2.0; and b. about 80 toabout 90 wt. percent based on the total composition of a filler selectedfrom the group consisting of electrically conductive metals and theiralloys, said filler having a particle size range of about 0.05 to about200 microns.
 2. a filler selected from the group consisting ofelectrically conductive metals and their alloys, said filler having aparticle size range of about 0.05 to about 50 microns, said particleshaving an approximately linear distribution of particle sizes with anaverage particle size of about 1 to about 30 microns, wherein said firstinsulating semi-conductor coating comprises about 70 to about 90 wt.percent based on the total composition of filler; b. a series ofadditional layers of semi-conductor composition laid one upon the otherover said first layer, said additional layers comprising the compositionof (a) (1) and (2); each of said additional layers containing about 10to about 30 wt. percent less filler than the preceding layers; and c. afinal outer layer of said semi-conductor composition containing about 20to about 40 wt. percent filler.
 3. The composition of claim 2 whereinthe filler is present at about 90 wt. percent and has a particle sizerange of about 0.05 to about 50 microns, said particles having a lineardistribution of particle size with an average size of about 1 to about30 microns.
 4. An insulated conductor which comprises an electricallyconductive conductor insulated with a semi-conductor coating compositioncomprising: a. a first layer of the semi-conductor compositioncomprising: